1. Field of the Invention
This invention generally relates to fiber optic spectroscopy and, more specifically, to fiber optic laser-induced breakdown spectroscopy (LIBS) sensors for molten material analysis.
2. Description of Related Art
Laser-induced breakdown spectroscopy is an advanced diagnostic technique for measuring the concentration of various elements in a test medium. This technique works for solids, liquids and gases. A pulsed laser beam is typically used to generate a spark (high temperature plasma) comprising excited neutral atoms, ions, and electrons. The laser generated plasma is allowed to equilibrate, and the emission from the neutral and ionized atoms is collected and dispersed by a spectrograph fitted with an intensified charge coupled detector. The intensity of the emission lines in the spectrum is analyzed to deduce the elemental concentrations in the sample.
In early LIBS experiments, high power laser beams were focused onto the surface of a sample using a system of lenses to generate a spark (plasma). Another assembly of lenses at a right angle to the laser beam then collected the spark light. Photomultiplier tubes with boxcar averagers, photodiode arrays with multichannel analyzers and, more recently, intensified charge coupled devices were used to record the emission signals. These LIBS experimental setups, however, are not well suited for industrial/field measurements where access to test facilities is limited and on-site alignments are difficult to complete.
Recent advances in fiber optic materials have opened up many new areas of application for the LIBS technique. Through a beam-delivery system, a laser beam may be sent to a desired location and used to perform remote measurements with an optical fiber. To generate plasma on the surface of a solid or liquid, a very high-powered laser beam is required. Thus, a difficult task in designing an optical fiber LIBS probe is coupling a high-energy laser beam into an optical fiber without damaging the fiber. Due to the breakdown threshold of the optical fiber material, optical fibers in LIBS were initially limited to delivering emission signals to the detection system. For example, a fused silica incoherent fiber bundle was used instead of a lens to collect the emission signal from the laser spark. A multiple optical fiber system, with each fiber pointing at a different region of the spark, has also been used.
Recently, more LIBS investigations using two optical fibers have been reported, one optical fiber for delivering the laser to create sparks on the surface of the sample and another optical fiber for collecting emission signals from the spark. A feature of LIBS is it can be used to perform measurements in harsh and hazardous environments, such as those in the aluminum, glass, and steel industries. Nevertheless, adjustment of the two optical fibers, one for launching the laser radiation and the other for collecting emission signals from the spark, is a very delicate and difficult task. Therefore, it is desirable to use one optical fiber both for transmitting a laser beam and for collecting emission signals from the laser-induced plasma.
U.S. Pat. No. 5,085,499 to Jeffrey, et al., discloses a method of on-site monitoring of a body of fluid stored in a tank or groundwater using an electrical spark and a fiber optic system to collect the atomic emission from the spark. This probe may be suitable for conducting materials, but may not be suitable for non-conducting materials or for studying the materials at high temperature (e.g., molten material).
U.S. Pat. No. 5,185,834 to Leslie, et al., generally discloses an optical fiber probe used with a spectrophotometer in a remote analysis system.
U.S. Pat. No. 5,798,832 to Klaus, et al., discloses a device for reducing the effects of changing sample surface by using a measuring head comprising a casing having radiation optics. The measuring head is mounted at a defined constant distance from the exit opening of the casing, so that the focal point of the laser beam is in the plane of the exit opening. The device is suitable for analyzing and compensating different surface contours or different dimensions of solid samples.
U.S. Pat. No. 5,128,882 to Cooper, et al., discloses a fiber optic cone penetrometer probe to irradiate the soil with ultraviolet (UV) or visible light to generate a fluorescence, reflection, or absorption spectrum of soil contaminates. The fluorescence spectroscopy described in this patent provides information for classifying certain molecular species, but does not form a plasma and is generally insensitive to atomic species that are important to the analysis of aluminum and steel alloy.
U.S. Pat. No. 5,379,103 to Ziger, et al., discloses a dual mode probe including separate optical fibers to conduct excitation and to collect response signals. Using separate optical fibers to conduct excitation and to collect response signals is impractical for molten materials analysis because adjustment of the two optical fibers is a very delicate and difficult task.
U.S. Pat. No. 6,147,754 to Gregory, et al., discloses a LIBS cone penetrometer in which one fiber optic carries an excitation signal from an energy source (laser), and a response signal from the sample surface is back transmitted via the same fiber. To separate the reflected excitation signal and the response signal, a decoupling mirror (a surface polished metallic mirror) was used to reflect the response signal. The mirror has a center hole of a diameter xe2x85x9xe2x80x3 that allows the excitation signal to pass through. Drawbacks of this device include: (1) a part of the response signal, which coincides with the excitation signal, passes through the hole in the mirror and is lost; (2) a very small focal length lens (4 mm) is used to focus the laser beam on the surface of the sample, thus allowing small variations in the sample surface to introduce inconsistency in successive measurements; (3) the probe is not suitable for high temperature analysis; and (4) the atomic emission reflecting from the decoupling mirror is focused directly on the spectrometer, limiting the probe to certain remote measurements.
Fiber optic LIBS probes have made progress in the analysis of solid and gaseous materials; however, much work remains to be done in the development of fiber optic LIBS sensors in the study of molten materials.
The present invention includes a fiber optic LIBS sensor providing on-site, online, and real-time measurement of elemental composition of molten alloy in a furnace. This sensor functions as a process monitor and control tool for the aluminum, steel, and glass industries. The sensor measures the transmission of laser energy through a multimode optical fiber. The laser radiation from the fiber is collimated and focused on an aluminum melt in the furnace, with a specially designed high temperature lens holder, such as a stainless steel lens holder, that holds the collimating and focusing lenses. Atomic emission from the plasma is collected by the same lenses and back-transmitted through the same optical fiber, and then sent to the spectrograph via an optical fiber bundle.
The fiber optic LIBS sensor further provides enhanced product quality control, saving time, and improving efficiency of the glass and metal melting processes.
The features and advantages summarized above in addition to other aspects of the present invention will become more apparent from the description, presented in conjunction with the following drawings.